[chore][pkg/translator/faro] run make modernize (open-telemetry#44261)

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#### Description

Run the `make modernize` tool.

https://pkg.go.dev/golang.org/x/tools/gopls/internal/analysis/modernize

This is part of the process to enable the
[modernize](open-telemetry#44199)
linter in CI.

There are no changes in the component behaviour.

```sh
pkg/translator/faro/keyval.go:122:4: stringsbuilder: using string += string in a loop is inefficient (modernize)
			stacktrace += frameToString(&e.Stacktrace.Frames[i])
			^
```

Signed-off-by: Paulo Dias <[email protected]>

Author: paulojmdias

pkg/translator/faro/keyval.go

@@ -116,13 +116,14 @@ func exceptionMessage(e *faroTypes.Exception) string {
 
 // exceptionToString is the string representation of an Exception
 func exceptionToString(e *faroTypes.Exception) string {
-	stacktrace := exceptionMessage(e)
+	var stacktrace strings.Builder
+	stacktrace.WriteString(exceptionMessage(e))
 	if e.Stacktrace != nil {
 		for i := range e.Stacktrace.Frames {
-			stacktrace += frameToString(&e.Stacktrace.Frames[i])
+			stacktrace.WriteString(frameToString(&e.Stacktrace.Frames[i]))
 		}
 	}
-	return stacktrace
+	return stacktrace.String()
 }
 
 // frameToString function converts a Frame into a human readable string